Dietary supplementation with 1.5 g to 2.0 g elemental calcium per day during pregnancy has been found to reduce blood pressure levels and the frequency of preterm birth in two modest randomized trials. It is proposed to test this effect in a larger 2-3 center collaborative, double-masked, randomized clinical trial in which about 4000 women will be enrolled in the first or second trimester and will take 4 calcium tablets (or placebo tablets) daily. Standardized blood pressure measurements will be obtained at entry, at routine prenatal visits and at least once near term. Urinary protein excretion and pregnancy dating will be carefully recorded. Primary outcomes of the trial will be the frequency of pregnancy induced hypertension and of preterm birth. Birth weight will be an important secondary outcome. Proposal reviews have been conducted, and contracts are expected to be awarded in October, 1990.